Fixed SDL_PRI?64 for 64-bit Linux.

64-bit Linux uses a "long" instead of "long long" for 64-bit ints.

Added a special-case this so SDL_PRI?64 doesn't trigger compiler warnings
when used with SDL's 64-bit datatypes on 64-bit Linux.

include/SDL_stdinc.h

@@ -173,6 +173,8 @@ typedef uint64_t Uint64;
 #define SDL_PRIs64 PRIs64
 #elif defined(__WIN32__)
 #define SDL_PRIs64 "I64d"
+#elif defined(__LINUX__) && defined(__LP64__)
+#define SDL_PRIs64 "ld"
 #else
 #define SDL_PRIs64 "lld"
 #endif
@@ -182,6 +184,8 @@ typedef uint64_t Uint64;
 #define SDL_PRIu64 PRIu64
 #elif defined(__WIN32__)
 #define SDL_PRIu64 "I64u"
+#elif defined(__LINUX__) && defined(__LP64__)
+#define SDL_PRIu64 "lu"
 #else
 #define SDL_PRIu64 "llu"
 #endif
@@ -191,6 +195,8 @@ typedef uint64_t Uint64;
 #define SDL_PRIx64 PRIx64
 #elif defined(__WIN32__)
 #define SDL_PRIx64 "I64x"
+#elif defined(__LINUX__) && defined(__LP64__)
+#define SDL_PRIx64 "lx"
 #else
 #define SDL_PRIx64 "llx"
 #endif
@@ -200,6 +206,8 @@ typedef uint64_t Uint64;
 #define SDL_PRIX64 PRIX64
 #elif defined(__WIN32__)
 #define SDL_PRIX64 "I64X"
+#elif defined(__LINUX__) && defined(__LP64__)
+#define SDL_PRIX64 "lX"
 #else
 #define SDL_PRIX64 "llX"
 #endif